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<?php
/**
* @file
* A database-mediated implementation of a locking mechanism.
*/
/**
* @defgroup lock Locking mechanisms
* @{
* Functions to coordinate long-running operations across requests.
*
* In most environments, multiple Drupal page requests (a.k.a. threads or
* processes) will execute in parallel. This leads to potential conflicts or
* race conditions when two requests execute the same code at the same time. A
* common example of this is a rebuild like menu_rebuild() where we invoke many
* hook implementations to get and process data from all active modules, and
* then delete the current data in the database to insert the new afterwards.
*
* This is a cooperative, advisory lock system. Any long-running operation
* that could potentially be attempted in parallel by multiple requests should
* try to acquire a lock before proceeding. By obtaining a lock, one request
* notifies any other requests that a specific operation is in progress which
* must not be executed in parallel.
*
* To use this API, pick a unique name for the lock. A sensible choice is the
* name of the function performing the operation. A very simple example use of
* this API:
* @code
* function mymodule_long_operation() {
* if (lock_acquire('mymodule_long_operation')) {
* // Do the long operation here.
* // ...
* lock_release('mymodule_long_operation');
* }
* }
* @endcode
*
* If a function acquires a lock it should always release it when the
* operation is complete by calling lock_release(), as in the example.
*
* A function that has acquired a lock may attempt to renew a lock (extend the
* duration of the lock) by calling lock_acquire() again during the operation.
* Failure to renew a lock is indicative that another request has acquired
* the lock, and that the current operation may need to be aborted.
*
* If a function fails to acquire a lock it may either immediately return, or
* it may call lock_wait() if the rest of the current page request requires
* that the operation in question be complete. After lock_wait() returns,
* the function may again attempt to acquire the lock, or may simply allow the
* page request to proceed on the assumption that a parallel request completed
* the operation.
*
* lock_acquire() and lock_wait() will automatically break (delete) a lock
* whose duration has exceeded the timeout specified when it was acquired.
*
* Alternative implementations of this API (such as APC) may be substituted
* by setting the 'lock_inc' variable to an alternate include filepath. Since
* this is an API intended to support alternative implementations, code using
* this API should never rely upon specific implementation details (for example
* no code should look for or directly modify a lock in the {semaphore} table).
*/
/**
* Initialize the locking system.
*/
function lock_initialize() {
global $locks;
$locks = array();
}
/**
* Helper function to get this request's unique id.
*/
function _lock_id() {
// Do not use drupal_static(). This identifier refers to the current
// client request, and must not be changed under any circumstances
// else the shutdown handler may fail to release our locks.
static $lock_id;
if (!isset($lock_id)) {
// Assign a unique id.
$lock_id = uniqid(mt_rand(), TRUE);
// We only register a shutdown function if a lock is used.
drupal_register_shutdown_function('lock_release_all', $lock_id);
}
return $lock_id;
}
/**
* Acquire (or renew) a lock, but do not block if it fails.
*
* @param $name
* The name of the lock.
* @param $timeout
* A number of seconds (float) before the lock expires (minimum of 0.001).
*
* @return
* TRUE if the lock was acquired, FALSE if it failed.
*/
function lock_acquire($name, $timeout = 30.0) {
global $locks;
// Insure that the timeout is at least 1 ms.
$timeout = max($timeout, 0.001);
$expire = microtime(TRUE) + $timeout;
if (isset($locks[$name])) {
// Try to extend the expiration of a lock we already acquired.
$success = (bool) db_update('semaphore')
->fields(array('expire' => $expire))
->condition('name', $name)
->condition('value', _lock_id())
->execute();
if (!$success) {
// The lock was broken.
unset($locks[$name]);
}
return $success;
}
else {
// Optimistically try to acquire the lock, then retry once if it fails.
// The first time through the loop cannot be a retry.
$retry = FALSE;
// We always want to do this code at least once.
do {
try {
db_insert('semaphore')
->fields(array(
'name' => $name,
'value' => _lock_id(),
'expire' => $expire,
))
->execute();
// We track all acquired locks in the global variable.
$locks[$name] = TRUE;
// We never need to try again.
$retry = FALSE;
}
catch (PDOException $e) {
// Suppress the error. If this is our first pass through the loop,
// then $retry is FALSE. In this case, the insert must have failed
// meaning some other request acquired the lock but did not release it.
// We decide whether to retry by checking lock_may_be_available()
// Since this will break the lock in case it is expired.
$retry = $retry ? FALSE : lock_may_be_available($name);
}
// We only retry in case the first attempt failed, but we then broke
// an expired lock.
} while ($retry);
}
return isset($locks[$name]);
}
/**
* Check if lock acquired by a different process may be available.
*
* If an existing lock has expired, it is removed.
*
* @param $name
* The name of the lock.
*
* @return
* TRUE if there is no lock or it was removed, FALSE otherwise.
*/
function lock_may_be_available($name) {
$lock = db_query('SELECT expire, value FROM {semaphore} WHERE name = :name', array(':name' => $name))->fetchAssoc();
if (!$lock) {
return TRUE;
}
$expire = (float) $lock['expire'];
$now = microtime(TRUE);
if ($now > $expire) {
// We check two conditions to prevent a race condition where another
// request acquired the lock and set a new expire time. We add a small
// number to $expire to avoid errors with float to string conversion.
return (bool) db_delete('semaphore')
->condition('name', $name)
->condition('value', $lock['value'])
->condition('expire', 0.0001 + $expire, '<=')
->execute();
}
return FALSE;
}
/**
* Wait for a lock to be available.
*
* This function may be called in a request that fails to acquire a desired
* lock. This will block further execution until the lock is available or the
* specified delay in seconds is reached. This should not be used with locks
* that are acquired very frequently, since the lock is likely to be acquired
* again by a different request while waiting.
*
* @param $name
* The name of the lock.
* @param $delay
* The maximum number of seconds to wait, as an integer.
*
* @return
* TRUE if the lock holds, FALSE if it is available.
*/
function lock_wait($name, $delay = 30) {
// Pause the process for short periods between calling
// lock_may_be_available(). This prevents hitting the database with constant
// database queries while waiting, which could lead to performance issues.
// However, if the wait period is too long, there is the potential for a
// large number of processes to be blocked waiting for a lock, especially
// if the item being rebuilt is commonly requested. To address both of these
// concerns, begin waiting for 25ms, then add 25ms to the wait period each
// time until it reaches 500ms. After this point polling will continue every
// 500ms until $delay is reached.
// $delay is passed in seconds, but we will be using usleep(), which takes
// microseconds as a parameter. Multiply it by 1 million so that all
// further numbers are equivalent.
$delay = (int) $delay * 1000000;
// Begin sleeping at 25ms.
$sleep = 25000;
while ($delay > 0) {
// This function should only be called by a request that failed to get a
// lock, so we sleep first to give the parallel request a chance to finish
// and release the lock.
usleep($sleep);
// After each sleep, increase the value of $sleep until it reaches
// 500ms, to reduce the potential for a lock stampede.
$delay = $delay - $sleep;
$sleep = min(500000, $sleep + 25000, $delay);
if (lock_may_be_available($name)) {
// No longer need to wait.
return FALSE;
}
}
// The caller must still wait longer to get the lock.
return TRUE;
}
/**
* Release a lock previously acquired by lock_acquire().
*
* This will release the named lock if it is still held by the current request.
*
* @param $name
* The name of the lock.
*/
function lock_release($name) {
global $locks;
unset($locks[$name]);
db_delete('semaphore')
->condition('name', $name)
->condition('value', _lock_id())
->execute();
}
/**
* Release all previously acquired locks.
*/
function lock_release_all($lock_id = NULL) {
global $locks;
$locks = array();
if (empty($lock_id)) {
$lock_id = _lock_id();
}
db_delete('semaphore')
->condition('value', $lock_id)
->execute();
}
/**
* @} End of "defgroup lock".
*/
